Carbon Isotopes, Kerogen Types and the Permian-Triassic Boundary in Australia: Implications for Exploration

Abstract

A global stratotype section for the boundary between the Permian and Triassic Systems is yet to be agreed internationally. However, in many parts of the world, there is evidence of massive extinction of marine fauna at or near the P-T boundary. In the absence of marine fauna, as is the case in most of Australia, changes in plant microfossils, in carbon isotopic signatures, and radiometric dates using SHRIMP, have been used to mark the boundary. The leading questions which arise from this are whether these events are synchronous and how they affect exploration.In eastern Australia, the top of the coal measures has been used as the top of the Permian, although presently only plant microfossils can be used to determine if the youngest seam is present. This has important economic consequences for determination of volumes of effective source rock and for estimates of coal-bed methane. It has been suggested that changes in the carbon isotopic signatures (δ13C) of either organic matter (δ13Corg) or carbonates (δ13Ccarb) can be used to delimit the P-T boundary. From studies of (δ13Ccarb data, Scholle (1995) concluded that 'the establishment of a secular variation curve which has enough detail and reliability to be used as a chemostratigraphic tool still lies in the future'. We concur, and demonstrate for organic carbon that, while there is evidence for secular change, the 13C signature is complicated and overprinted by contributions from different parent plant types; wood-dominated organics are typically −24 %o, while non-woody matter is significantly lighter (−30 %o). There is no simple relationship of δ13Corg to geological age. The consistency of our data, from both the east and west of Australia, allows us to construct a model to predict probable depositional and organic facies using either carbon isotopes (from kerogen or oil) or quantitative estimates of organic matter from standard palynlogical slides. This offers a novel and usefl predictive tool for hydrocarbon exploration.

Geology of the Permian Kuriwao Group, Murihiku Terrane, Southland, New Zealand

“…In recent years there has been a tendency, partly on the basis of carbon isotope abundance variations (e.g., Morante et al 1994;Morante 1996), to place it within the P. microcorpus Zone (sensu Helby et al 1987). But Foster and coworkers (e.g., Foster et al 1997Foster et al , 1998 have concluded that these carbon isotope changes in part reflect fluctuating relative abundances of woody and non-woody derived kerogen, which in turn are due to changes in sedimentary facies that are not necessarily globally correlative. We concur with Foster et al (1997Foster et al ( , 1998 in discounting the correlative value of the carbon isotope data.…”

Geochemical Data on Marine Sediments of the Permian-Triassic Boundary Interval: A Review

“…Foster et al (1997) determined facies and organic models based on isotopic and chemical composition of organic carbon from E and W Australian basin examples.…”

“…At high paleolatitude of the Southern hemisphere, in Western and Eastern Australian, carbon isotope profiles also exhibit a high magnitude drop in δ 1 3 Corg values. However, Foster et al (1997) concluded that the isotopic values of organic carbon are strongly influenced by the primary source of the organic matter and questioned their use in chemostratigraphic correlations.…”

Organic carbon isotope values from the Late Permian Seis/Siusi succession (Dolomites, Italy): Implications for palaeoenvironmental changes

“…For instance, Permian wood shows heavier carbon isotope values than coeval marine organic matter (Faure et al 1995;Foster et al 1997;Korte et al 2001). Changes in the amount of terrestrial organic matter in marine sediments probably impacted the negative carbon-isotope shift in Australian PTB successions (Morante 1996;Foster et al 1997;Thomas et al 2004).…”

“…For instance, Permian wood shows heavier carbon isotope values than coeval marine organic matter (Faure et al 1995;Foster et al 1997;Korte et al 2001). Changes in the amount of terrestrial organic matter in marine sediments probably impacted the negative carbon-isotope shift in Australian PTB successions (Morante 1996;Foster et al 1997;Thomas et al 2004). To complicate matters, ocean anoxia was widespread in the latest Permian oceans (e.g., Wignall & Hallam 1992;Kajiwara et al 1994;Isozaki 1997;Algeo et al 2008;Grasby & Beauchamp 2009) reaching even shallow water depths, as traced by green sulphur bacteria (¼ Chlorobiaceae) in the PTB Hovea-3 core from W-Australia (Grice et al 2005).…”